1. Gas Reactions Page 4 2. Measuring the Speed Page 6 3. Increasing the Speed Page Making Foam Page Putting Out a Fire Page 18

Transcription

1 P & L Johnson 2012

2 A foam fire extinguisher puts out fires by producing a blanket of foam that contains carbon dioxide rather than air. This smothers the fire preventing oxygen getting to the fuel. In order to produce your own foam fire extinguisher you will need to find a reaction that produces carbon dioxide and learn to produce a foam. You will also have to find a way to keep the chemicals separate until you need to produce the foam. You will need work out how to make lots of gas in a very short time. To do this you will need to have a good understanding of how the speed of a reaction can be measured and altered. 1. Gas Reactions Page 4 2. Measuring the Speed Page 6 3. Increasing the Speed Page Making Foam Page Putting Out a Fire Page 18 P & L Johnson

3 Nat 4 outcomes O Nat 5 outcomes By the end of this unit you should know the following: How to calculate the average rate of a reaction using: average rate = change in property measured time How to follow a chemical reaction by measuring the change in mass or volume of gas produced. How to plot a graph of mass of gas against time and use the graph to calculate the average rate of a reaction. How changing the size of the reactant particles, concentration of solutions or the temperature affects the speed of a reaction. How changing the particle size, concentration or temperature affects the shape of a graph following a reaction. Be able to explain how changing the particle size, concentration or temperature speeds up a chemical reaction. P & L Johnson

4 Testing Gases In order to produce a foam fire extinguisher you will need to be able to produce carbon dioxide gas. There are many reactions that produce gases but not all produce carbon dioxide; some produce hydrogen and others oxygen. It is therefore important that we can identify carbon dioxide when it is produced. ACTIVITY 1.1 Testing Gases Your teacher will show you how to test for the gases: oxygen, hydrogen and carbon dioxide. DISCUSS After discussion with your teacher and others can you describe the tests for the gases oxygen, hydrogen and carbon dioxide. Complete the following table: Gas Oxygen Hydrogen Carbon dioxide Result of a Positive Test Relights a glowing splint. Burns with a squeaky pop. Turns limewater cloudy (or chalky). ACTIVITY 1.2 Making Gases Your teacher will give you a number of solutions and solids. You will need to work out which combinations will produce carbon dioxide gas. You are only allowed to react one solution with one solid. The table below shows you the chemicals that you can use. Solids Solutions Magnesium Sodium hydrogencarbonate Manganese dioxide Hydrochloric acid Citric acid Hydrogen peroxide Marble chips P & L Johnson

5 Record your results, for those that produced a gas, in the following table: Reaction Magnesium + Hydrochloric Acid Sodium hydrogencarbonate + Hydrochloric Acid Manganese dioxide + Hydrogen peroxide Marble chips + Hydrochloric Acid Magnesium + Citric Acid Sodium hydrogencarbonate + Citric Acid Marble chips + Citric Acid Name of Gas Produced Hydrogen Carbon Dioxide Oxygen Carbon Dioxide Hydrogen Carbon Dioxide Carbon Dioxide P & L Johnson

6 As well as knowing the best combination of chemicals to produce the carbon dioxide we also need to know how fast the carbon dioxide is being produced so that we can work out whether any changes we make to our system are causing it to produce the gas more quickly. The speed of a car is usually given in km per hour or miles per hour, this is really the distance travelled in one hour. If a car had a speed of 60 miles per hour it could travel 60 miles in one hour or 30 miles in half an hour or 120 miles in two hours. So distance = speed x time Or speed = distance time The speed or rate of a chemical reaction can be measured in a similar way. In a reaction that produces a gas, the mass of the container in which the reaction takes place will decrease as the gas escapes. We can use the initial mass of the reaction container to work out the mass of gas produced. So rate = mass of gas produced time In order to work out the rate of a reaction we need to know how much gas is being lost over the time the reaction goes on for. ACTIVITY 1.3 Measuring the Mass of Gas Produced 1. Pour 50cm 3 of 1moll -1 HCl into a conical flask, put a piece of cotton wool in its mouth and place on a balance. 2. Using a small beaker, weigh out 10g of marble chips and place it on the balance as well. 3. Record the initial mass in the following table. 4. Take out the cotton wool and pour in the marble chips, start the stop watch and replace the cotton wool. 5. Measure the mass every 30 seconds and put your results in the table. P & L Johnson

7 Time (min) Mass of container (g) Mass of gas produced (g) Time (min) Mass of container (g) Mass of gas produced (g) Fill in the table with your results, then calculate the mass of gas produced, show a sample calculation in the space below: P & L Johnson

8 Mass of Gas Produced / g Use your results from the previous page to plot a graph of Mass of Gas against time Time / min You can see from your graph that the steepness of the curve changes with time. This is because the speed of the reaction is always changing. You can use your graph to work out the speed of the reaction. Your teacher will show you how to read from the graph to calculate the average speed of the reaction at certain time intervals. Average rate = change in mass time interval DISCUSS After discussion with your teacher and others can you calculate average rates of reaction for various time intervals from your graph? Use your graph to calculate the average rate of reaction over the following time intervals: a. 0-2 minutes c. 4-6 minutes b. 2-4 minutes d minutes P & L Johnson

9 There are a number of ways that reactions can be followed and the changes that take place over time can be used to calculate the average rate of the reaction. DISCUSS After discussion with your teacher and others can you think of other ways that the reaction between marble chips and hydrochloric acid can be followed. In the space below write down your ideas and draw diagrams to show different ways you would follow the reaction between marble chips and hydrochloric acid: P & L Johnson

10 Quick Test 1 1. In a reaction, 60cm 3 of gas was collected in 20 s. The average rate of reaction in cm 3 s -1, was A B C D The graph below shows the variation of concentration of a reactant with time as a reaction proceeds. During the first 25s, the average rate of reaction in moll -1 s -1, is A B C D During the first 20 seconds of a reaction, 5.0cm 3 of gas was given off. The average rate of reaction in cm 3 s -1, during the first 20 seconds is A B. 5.0 C. 4.0 D P & L Johnson

11 In order for a reaction to occur reactant particles must collide. To speed up a reaction we therefore need to increase the number of collisions taking place, the more collisions taking place, the more products can be produced. In a typical reaction involving a solid and a solution we can imagine the reaction mixture looking something like this: Water in the solution Reactant moving in the solution Solid reactant Only when two reactant molecules collide with one another can a reaction occur and this can only occur on the outside of the solid. DISCUSS After discussion with your teacher and others can you think of ways that you could increase the number of collisions taking place in the reaction mixture. In the space below write down your ideas: Increase temperature or concentration. Make the particles smaller. P & L Johnson

12 Increasing Surface Area A reaction s speed can be altered by increasing the surface area of one of the reactants. This can be achieved by breaking up lumps into chips or even better grinding it into a powder. When this occurs more of the reactant s particles are exposed and therefore more particles can come in contact with the other reactant. This means that there will be a greater number of collisions, resulting in a faster rate of reaction. ACTIVITY 1.4 Effect of Surface Area After looking at the available chemicals and equipment and discussion with others in your class and your teacher, devise an experiment that would show clearly and fairly that increasing the surface area of a solid reactant speeds up the reaction. Increasing Concentration A reaction s speed can be altered by increasing the concentration of a solution or pressure in a gas. When this occurs there are more reactant particles in a given volume of space and therefore more particles can come in contact with the other reactant. This means that there will be a greater number of collisions, resulting in a faster rate of reaction. ACTIVITY 1.5 Effect of Concentration After looking at the available chemicals and equipment and discussion with others in your class and your teacher, devise an experiment that would show clearly and fairly that increasing the concentration of a reactant solution speeds up the reaction. P & L Johnson

13 Increasing Temperature A reaction s speed can be altered by increasing the temperature of the reaction mixture. When this occurs all the reactant particles have more energy and therefore more speed so a much greater number will collide more often and with more energy. This means that there will be a greater number of collisions, resulting in a faster rate of reaction. ACTIVITY 1.6 Effect of Temperature After looking at the available chemicals and equipment and discussion with others in your class and your teacher, devise an experiment that would show clearly and fairly that increasing the temperature speeds up the reaction. In the space below briefly describe the three investigations and your conclusions: When temperature was increased, more gas was released more quickly, showing that the reaction was faster at higher temperatures. The slope of the line was steeper at higher temperatures but at the end the same volume of gas was collected. The particles collide more frequently and with more energy when temperature is higher. When concentration was increased, more gas was released more quickly, showing that the reaction was faster at higher concentrations. The slope of the line was steeper at higher temperatures but at the end the same volume of gas was collected. The particles collide more frequently when concentration is higher. When larger particles were used, less gas was released more slowly, showing that the reaction was slower for bigger particles The slope of the line was steeper for smaller particles but at the end the same volume of gas was collected. The particles collide more frequently when particles are smaller because surface area is larger. P & L Johnson

14 Shapes of Graphs During the course of a reaction the steepness of the curve changes, getting less steep as reactants are used up and the speed of reaction slows down. For a reaction producing a gas the shape will tend to look like graph A: graph A However the steepness of the curve will change if we alter either the surface area of the solid, concentration of acid or the temperature. Graph B shows the shape if the surface area, concentration or temperature are increased: graph B Graph C shows the shape if the surface area, concentration or temperature are decreased: graph C Note the final mass or volume of gas can change if there is more or less reactants. This is often the case when the concentration of the acid changes and the solid is in excess. P & L Johnson

15 Quick Test 2 1. Which of the following pairs of reactants would produce hydrogen most slowly? A. Magnesium powder and 4 moll -1 acid. B. Magnesium ribbon and 2 moll -1 acid. C. Magnesium powder and 2 moll -1 acid. D. Magnesium ribbon and 4 moll -1 acid. 2. Graph P shows the volume of hydrogen gas when 1.0g of magnesium ribbon reacts with excess 2 moll -1 hydrochloric acid. Which of the following samples of magnesium would react with the acid to produce graph Q? A. 0.5g of magnesium ribbon B. 0.5g of magnesium powder C. 1.0g of magnesium powder D. 2.0g of magnesium ribbon 3. The table shows the times taken for 0.5g of magnesium to react completely with acid under different conditions. The time taken for 0.5g of magnesium to react completely with 0.2moll -1 acid at 25 0 C will be A. less than 10 s B. between 10 s and 20 s C. Between 20 s and 60 s D. More than 80 s. P & L Johnson

16 DISCUSS After discussion with your group plan how you are going to produce your carbon dioxide gas as quickly as possible. You will need to consider the following points: 1. What chemicals you will use to produce the carbon dioxide gas. 2. How are you going to achieve the fastest rate possible? Remember to be safe and practical, also your chemicals will be stored inside the fire extinguisher and be left at room temperature most of the time. In the space below write down your ideas:from lesson one, we know that carbon dioxide can be made by adding a carbonate to an acid like hydrochloric acid or citric acid. An acid kept separate from the solid carbonate until needed could then be mixed to produce bubbles of carbon dioxide foam to extinguish the fire. Hydrogen or oxygen gas shouldn't be used. Hydrogen is flammable and oxygen would increase the rate of burning. P & L Johnson

17 The easiest way to make foam is simply to add soap or detergent to water and shake it or stir it vigorously. But which soap or detergent will produce the best foam and are there any other ways to make foam? Research Task After discussion with your teacher and others can you find out how foams are made in commercial foam fire extinguishers. Shaving foam and hair styling mousse are also foams, can you find out how these are made. ACTIVITY 1.8 Making Foam After looking at the available chemicals and equipment and discussion with others in your class and your teacher identify the best way of producing lots of foam. In the space below write down your findings: Add a detergent like soap or Fairy Liquid, and produce the gas one the detergent is added. P & L Johnson

18 In order to put out a fire, a foam fire extinguisher needs to produce the carbon dioxide foam very quickly but must also not produce foam until it is needed. Therefore when designing the fire extinguisher you will need to keep your gas generating chemicals apart so that only when you want to produce the gas do they come together. You will also need to have your foam producing chemicals able to come in contact with the gas as it is produced. Finally you will need a way of directing the foam at the fire in order to smother it. DISCUSS After discussion with your group plan how you are going make your foam fire extinguisher. You will need to consider the following points: 1. How will you keep your gas generating chemicals apart? 2. How will you ensure the foam producing chemicals come in contact with the gas when it forms? 3. How will you direct the foam at a fire? In the space below write down your ideas and draw a diagram of your final design: P & L Johnson

19 If you are struggling the following picture may give you some ideas: ACTIVITY 1.9 Foam Fire Extinguisher Your teacher may let you make your own foam fire extinguisher and use it to put out a real fire. Research Task After discussion with your teacher and others, can you produce an advertising pamphlet for your foam fire extinguisher, with photographs and/or diagrams of how it works. P & L Johnson